A photo CVD apparatus includes a reaction chamber, a vacuum pump, and a light source chamber disposed in the reaction chamber, the light source chamber having a light window. A light source is provided in the light source chamber for irradiating the inside of the reaction chamber through the window. A device inputs reactive gas into the reaction chamber, and an electrode is disposed in the reaction space adjacent to the window and located between the substrate and the window. A method of depositing a layer on a substrate includes the steps of disposing a substrate in a reaction chamber, introducing a reactive gas, and initiating a photo-chemical reaction to deposit the product of the reaction on the substrate by irradiating the reactive gas with light emitted from a light source through a window of a light source chamber in which the light source is disposed. The substrate is then removed from the reaction chamber, and an etchant gas is introduced into the reaction chamber. A voltage is then applied between the substrate holder and an electrode which is located between the window and the substrate holder for carrying out plasma etching of the window.